The invention relates to a device for suppressing arcing in cathode sputtering installations in which at least one electrode is connected to an AC voltage source.
In cathode sputtering for the purpose of producing thin layers with the aid of medium-frequency techniques in which work takes place in a frequency range of a few hundred Hz to a few hundred kHz, arc discharges can be generated which can damage or destroy the layers to be produced as well as also the targets used in sputtering. This applies especially to the production of dielectric layers by means of reactive gas techniques in which into the receptacle a reactive gas is introduced with which the sputtered target material can react on the substrate. For example, to sputtered aluminum is added O.sub.2 as the reactive gas in order to produce a thin layer of Al.sub.2 O.sub.3.
Numerous devices have already been suggested with which it is possible to recognize arc discharges, to interrupt them or even to prevent their generation (German Patent Applications 42 30 779, 42 39 218, 42 42 633, and U.S. Pat. Nos. 4,931,169, 5,192,894, 5,281,321, 5,286,360, and 5,415,757. However, these devices are either not suitable for use in installations operating with AC voltages or they are not suited for recognizing and suppressing flashovers in the "germination phase", i.e. micro-flashovers or microarcs.
Micro-flashovers or microarcs which can be observed with an oscilloscope occur only over a halfwave or over a few halfwaves of the applied AC voltage. At an AC voltage frequency of 40 kHz this means that the arcs occur only for approximately 12.5 .mu.s or a multiple thereof. In particular with readily melting sputtering targets, such as for example Al, these microarcs lead to damage of the layer to be produced, since the energy present in the microarcs is already sufficient to melt droplets having a diameter of a few .mu.m off the target material and to sputter them onto the substrate. As a rule, the longer the duration of the microarc the more severe the damage.
Micro-discharges occur also as silent discharges in dielectrics and, specifically, in irregular sequences and with a duration in the range of nanoseconds. Their distribution in terms of space and time is statistical. Each micro-discharge comprises a thin nearly cylindrical canal with a pinched electrode spot on the metal electrode (B. Eliasson, M. Hirth and U. Kogelschatz: Ozone synthesis from oxygen in dielectric barrier discharges, J. Phys. D.: Appl. Phys. 20 (1987) 1421-1437). The silent discharge is preferably used in plasma chemistry to produce, for example, ozone (B. Eliasson, U. Kogelschatz: Nonequilibrium Volume Plasma Chemical Processing, IEEE Transactions on Plasma Science, Vol. 19, No. 6, 1991, pp. 1063-1077). The micro-discharges according to the invention, however, refer to discharges in plasmas.
Microarcs are typical for medium-frequency sputtering and, as a rule, represent precursor stages of large arcs. But this does not mean that every microarc must by necessity lead to a large arc. Depending on the target state, microarcs occur with greater or lesser frequency and it is not possible to even detect these microarcs with conventional methods.
A device for detecting micro-flashovers or microarcs has already been suggested, which comprises a counter for microarcs (non-published German Patent Application P 44 20 951.7). In this device a substrate to be coated is disposed opposing at least one sputtering cathode and one electrode is provided with a target. This electrode is connected to a medium-frequency source. By means of a device for recognizing micro-flashovers these micro-flashovers are detected and counted in a counting device. In the presence of a given number of micro-flashovers or in the presence of a given frequency of micro-flashovers, measures are taken to prevent large flashovers.